THIS IS A RUSH TRANSCRIPT. THIS COPY MAY NOT BE IN ITS FINAL FORM AND MAY BE UPDATED.

BLITZER: It's a very, very worrisome development, NATO now investigating a cross-border clash between two of its own helicopters in Afghanistan and ground forces in neighboring Pakistan. It comes at a time of extreme tension between Pakistan and the United States over the raid on bin Laden's compound.

CNN's Brian Todd is working this story for us.

Brian, what do we know about this clash?

BRIAN TODD, CNN CORRESPONDENT: Wolf, we have information that NATO aircraft and Pakistani forces both fired shots in this incident. Whether they fired them directly and purposefully at each other remains a bit unclear.

But we are at a point where the anger and mistrust between the two sides are at a boil.

(BEGIN VIDEOTAPE)

TODD (voice-over): Pakistan's tensions with the West ratcheted up to military hostility -- NATO helicopters engaged in an exchange involving Pakistani forces near the border between Pakistan and Afghanistan.

Two Pakistani soldiers were wounded, and Pakistan's military has launched what it calls a strong protest.

(on camera): A NATO official says the incident occurred when coalition helicopters supporting Forward Operating Base Tillman, which is inside Afghanistan, but just a few miles from the Pakistan border, were flying in Eastern Afghanistan.

One official says this base had been receiving intermittent fire from across the border. A NATO official says the helicopters were fired on twice from the Pakistani side, and after the second time, they fired from across the border into this North Waziristan area of Pakistan.

(voice-over): Pakistani intelligence officials offer a different version. They say the incident started when a NATO fighter jet entered Pakistani air space. The official said Pakistani troops fired on the jet from ground positions. They say the jet then retreated, returned with the NATO helicopters, and fired on a Pakistani position. That area is a flash point of militant activity and recent strain between the U.S. and Pakistan.

JEFFREY DRESSLER, INSTITUTE FOR THE STUDY OF WAR: The majority of the drone strikes in Pakistan occurred pretty much in this area here, which is home to both al Qaeda and Lashkar-e-Taiba, who are responsible for the Mumbai attacks, but also the Hakani network.

TODD: The Hakani network is a militant group allied with the Taliban. It's not clear whether the NATO aircraft were American. I asked analyst Jeffrey Dressler if Pakistani and western forces might have a hair trigger in their current state of agitation. (on camera) Is the build-up -- are the tensions between the two sides a direct byproduct of the raid of bin Laden's compound?

DRESSLER: I don't think they're a direct byproduct. What we do know is that the army is feeling particularly tense right now, the Pakistani army. Intentions are running very high. But, you know, I doubt that that was actually the rationale behind the incident that occurred at the border.

(END VIDEOTAPE)

BLITZER: Dressler does point out that there have been deadly cross-border incidents in the recent past before that bin Laden raid. In September of last year, NATO helicopters crossed into Pakistani air space from Afghanistan, chasing after insurgents. Dozens of people were killed in that incident, and Pakistan called that a breach of U.N. rules for foreign forces.

As we mentioned earlier, NATO says that it is investigating this incident -- Wolf.

BLITZER: Brian, NATO helicopters patrolling that area of the border, what's their mission?

TODD: Analysts say that's an area where the militants sneak across the border into Afghanistan to launch attacks on coerced forces. Then they sneak back into Pakistan.

The coalition choppers and other forces are there to seal off that area. But it's always a flash point where skirmishes cross borders. As we mentioned that incident in September last year. And that's maybe how this escalated.